This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Addback of donor T cells following T cell-depleted stem cell transplantation (SCT) can accelerate immune reconstitution and be effective against relapsed malignancy. After haploidentical SCT, a high risk of graft-versus-host disease (GvHD) had essentially precluded this option. We have therefore been depleting donor T cells of alloreactive precursor cells, using a CD25 immunotoxin. Our method appeared effective and as few as 5 x 104/kg allodepleted haploidentical donor cells substantially accelerated anti-viral immune recovery in the recipient, without increasing GvHD (Blood Plenary Paper[1]). Administration of higher doses of cells will be necessary to obtain an antileukemic effect, and it is almost inevitable that in some patients, these doses of cells will be sufficient to trigger GvHD even after allodepletion. We therefore propose to increase the safety of our approach by incorporating a suicide gene, inducible caspase 9 (iCasp9), in the allodepleted T cells, permitting their destruction should administration have adverse effects. iCasp9 consists of a pro-apoptotic molecule, human caspase 9, joined to a drug-binding domain derived from human FK506-binding protein;addition of a small molecule synthetic drug leads to homodimerization of caspase 9, activation of the caspase pathway and apoptosis of the transduced cells within 24 hours. The dimerizer, AP1903, has successfully completed safety-testing in human volunteers. We have generated a retroviral vector encoding iCasp9 and a selectable marker (truncated CD19) to enable enrichment of transduced cells to >90% purity. We plan to infuse escalating doses of iCasp9-expressing allodepleted cells, starting at doses that are safe even for unmodified allodepleted cells. Our hypothesis is that any GVHD that develops at higher dose levels will respond to administration of AP1903 because of destruction of the gene-modified allodepleted cells If our hypothesis is correct, this approach will enable safe administration of larger and more effective doses of donor cells for optimal anti-tumor and anti-viral immune reconstitution in the majority, while ensuring that the minority who develop GVHD can be effectively treated.